1. Field of the Invention
The present invention relates to a keyboard device that identifies a key that is pressed among multiple keys, and more particularly, to a keyboard device that is used to provide user input to an electronic device, such as a mobile phone, personal digital assistant, or handheld computer.
2. Discussion of the Background
Electronic devices such as mobile phones and handheld computers have multiple input keys, through which a variety of information is input to the device.
The keyboard device part of such electronic devices identifies a pressed key from among the multiple keys thereof, and the electronic device performs a process in accordance with the input operation specified by the pressed key.
In a scan-type keyboard including multiple keys (key switches) arranged in the form of a matrix, that is, as an array of rows and columns, several approaches have been disclosed in an attempt to provide the ability to perform multiple different operations using only a limited number of keys.
Thus, for example, in one known method, the electronic device operates differently when the user touches only a single key and when the user touches multiple keys simultaneously in combination.
However, in the keyboard device having such a configuration, when multiple keys are pressed at the same time, the signal ordinarily generated thereby might be reversed or deflected like a leak, which is called a leakage current or a sneak current. Such a sneak current can cause a so-called “dummy input”, which is a phenomenon in which it appears as if a key other than the key which has actually been pressed. A detailed example of this phenomenon is described below.
A simplified key matrix of such type of keyboard device is shown in FIG. 5. Being controlled by a controller, not shown, the key matrix in which key switches SW1 through SW16 are arranged is supplied with scan signals shown as XSCAN1 to XSCAN4. The controller reads out receiving lines Y1 to Y4 via a readout receiving circuit, not shown, at a predetermined readout timing described below. The receiving signal lines Y1 to Y4 are provided with a power supply voltage via resistors R1 through R4.
Switches SW1 through SW4 are readout targets when a scan signal for XSCAN1 is output. Similarly, the switches SW5 through SW8 are readout targets when the scan signal for XSCAN2 is output. The switches SW9 through SW12 are the readout targets when the scan signal is output for XSCAN3, and switches SW13 through SW16 are the readout targets when the scan signal is output for XSCAN4.
When the three switches SW6, SW7, and SW11 are on at the same time, they are respectively read out while the scan signal is respectively being output for XSCAN2, XSCAN2, and XSCAN3.
The switches SW6 and SW7 can be read out correctly by the controller in combinations of the scan signal XSCAN3 and the receiving signal lines Y2 and Y3, respectively. However, subsequently, while the scan signal XSCAN3 is being output, the switch SW10 is read out together with the switch SW11 as being on even though the switch SW1 is not turned ON, because of a current flowing through a route “a” shown in FIG. 5 as a sneak current due to the scan signal SCAN3 and the receiving signal Y2.
Several approaches have been proposed to prevent such dummy input. In one known method, shown in FIG. 6, the keyboard device prevents the sneak and reverse current by including diodes D1 through D16 for preventing backflow of electrical current for all keys.
However, such a configuration, in which each key requires a diode, increases the number of parts, making the cost proportionally higher and proportionally increasing the size of the apparatus as well, neither of which is desirable.